IGN posts ANet's top 9 favorite cross profession combos.

Well of Darkness + Churning Earth = Rockin' AoE Damage
Professions: Necromancer, Elementalist
"This is a brutal combo. A necro can use Well of Darkness to create a Dark Field, which blinds enemies in the area. Along comes an elementalist attuned to earth, who cast Churning Earth to cripple enemies inside the Well of Darkness. When Churning Earth ends, it combines with the Dark Field to deliver additional AoE damage. Guaranteed to rock your enemies world – in a bad way."
-Ryan Diederich, Programmer

The Ranger -- capable of creating their own combos.

8
Contagion + Ricochet = Poison Axe to the Max
Profession: Ranger
"Rangers can choose some strange but useful pets, like the Murellow, which is kind of a diseased rat/bear that lives in the caves of Tyria. The unhygienic Murellow uses a skill called Contagion, which creates a Poison Field. An axe-wielding ranger can then use his basic attack, Richochet, to throw projectiles through the Poison Field. The axe ping pongs between enemies, applying Poison to a number of foes."
-Eric Flannum, Lead Game Designer

7
Smoke Screen + Stomp = Cloaks for Everyone!
Professions: Thief, Warrior
"This is one of my favorite combos. You take a thief, who uses his/her Smoke Screen skill to create a smoke field. Then a warrior comes along and blasts the smoke with a Stomp, adding Stealth to nearby allies by cloaking them in gloom. It's a very cool effect, and it comes in handy in combat."
-Ben Miller, Game Designer

6
Symbol of Swiftness + Big Ol' Bomb = Blindness Bomb
Professions: Guardian, Engineer
"A guardian can lay down a Symbol of Swiftness, which creates a Light Field on top of an engineer's Big Ol' Bomb that is about to explode. This will add Blindness to the explosion, effectively turning two offensive skills into an AoE defense!"
-Andrew McLeod, Game Designer

The Mesmer's clones and illusions use combos for a lot of their utility.

5
Healing Spring + Unload = Bullets that Heal
Profession: Ranger, Thief
"The skill Unload is one of the best ways to put out a ton of damaging projectiles, but when you combine it with a ranger's Healing Spring, you get a completely different effect. A ranger creates a Water Field with his Healing Spring skill, which a thief can then Unload through. These healing projectiles grants an area-of-effect heal to any allies that are attacking the thief's target. This healing doesn't apply to bad guys. This combo is one of the best ways to add some healing to an ally who is deep in hand to hand combat."
-Matt Pennebaker, Game Designer

4
Chaos Storm + Rapid Fire = Crazy Arrows
Profession: Mesmer, Ranger
"When a mesmer fires off Chaos Storm it grants random boons on allies and random conditions to any enemy within the effect area. You ranger ally inside the Chaos Storm can fire through the field, applying Confusion to each enemy he hits. Chaos!
-Jonathan Sharp, System Designer

3
Lava Font + Leaping Attack = A Burning Sensation
Profession: Elementalist, Mesmer/Warrior
"Here's how this one works. An elementalist triggers a Lava Font near an opponent. Then, your buddy the sword-wielding mesmer or warrior can leap into the Lava Font's area of effect, delivering additional burning damage to the enemy."
-Jon Peters, Game Designer

2
Frozen Ground + Eruption = Stay Frosty
Professions: Elementalist
"All you need is one elementalist for my favorite protective combo. Cast Eruption first, then attune to water and cast Frozen Ground. When the Eruption Bursts, damaging your foes, an Ice Field spreads over the area to apply a protective Frost Aura to you and your allies. I wish I could do this in real life on hot days."
-Karl McLain, QA Tester

1
Super Elixir + Illusionary Duelist = Condition Eraser
Professions: Engineer, Mesmer
"We call this getting Bearced! An engineer with Elixir Gun fires a ground targeted Super Elixir creating a Light Field. Then a mesmer with a pistol can summon an Illusionary Duelist to fire projectiles through the field, which will remove conditions from allies near the target."
-Tyler Bearce, QA Tester

I get the feeling that the combos aren't as specific as they're making them out to be. It seems more like any projectile through an ally's persistent aoe effect will do the same thing as any other projectile through the same effect. Same with Leaping Attack, Stomp, Eruption, Big Ol' Bomb (love that name, by the way), and so on into other persistent aoe effects - I'm thinking they'll all do the same thing, based on what persistent aoe effect they're done with.

I get the feeling that the combos aren't as specific as they're making them out to be. It seems more like any projectile through an ally's persistent aoe effect will do the same thing as any other projectile through the same effect. Same with Leaping Attack, Stomp, Eruption, Big Ol' Bomb (love that name, by the way), and so on into other persistent aoe effects - I'm thinking they'll all do the same thing, based on what persistent aoe effect they're done with.

No, they aren't really super specific, those are just some the cooler ones. Stomp on Frozen Ground and you get an Ice AoE. Frozen Ground on Eruption gives you an Ice AoE. Arrows through Fire/ Poison/ Lighting/ Chaos will do Fire/ Poison/ Lightning/ Chaos (confusion) damage.

i hope that not ALL of the CPC's are aoe-> trigger, some of the combos they talked about earlier in production (before the blog on CPC's) that worked on single targets seemed to be alot cooler (the launch -> impale combo as posted above)

i hope that not ALL of the CPC's are aoe-> trigger, some of the combos they talked about earlier in production (before the blog on CPC's) that worked on single targets seemed to be alot cooler (the launch -> impale combo as posted above)

its not the actual moves "launch" then "impale" it was basically whenever you used a certain move (i thought it was a Gsword warrior skill but looks like it wasnt, some kind of charging attack move) on a "launched" opponent (the effect not the skill) it would do extra damage

This is like the best parts of Magicka... minus the homicidal collateral damage...

what are you talkin about. the homicidal collateral damage was the best part of that game :P Intresting to finally have some examples of combos that go beyond the basic make fire patch and shoot through it. Most still fall along that general theme but those ones where two AOE effects detonate eachother have potential.

i hope that not ALL of the CPC's are aoe-> trigger, some of the combos they talked about earlier in production (before the blog on CPC's) that worked on single targets seemed to be alot cooler (the launch -> impale combo as posted above)

take a look at the blogpost again:

InitiatorsAll combos are initiated by creating an area in the world that changes some skills performed within it. We call these areas “fields” and they come in various flavors. They range from elemental effects—such as fire, ice, and lightning—to other effects like poison, light, dark, and smoke. All fields persist in the world for a time and can be taken advantage of by any number of finishers.

I would like to see you reacting to a launch fast enough to do a specific finisher in time... this launch-impale was only an example in wich direction they tried to go, but they could only work on this properly when most of the skills/classes were finished:

This system has been in development for some time. We knew it had a lot of potential, but we also knew we couldn’t flesh it out until more of the game was developed. Now that seven of our eight professions have been revealed (and the last one is undergoing testing), we have started to flesh out this system and create the building blocks that make it work.

So many times yes. I'm very excited about this combo system. The support options are what really interest me, because not having dedicated healers is one thing I think many people will have trouble adjusting to at first. In my last year of WoW, just before Cata and up to 4.2, I played a warrior tank and an rl friend of mine was the healer. We did many, many LFD groups and the overall attitude of many dps was, 'I'll just attack and it's up to the tank and healer to keep up with me.' This system will really encourage much more communication than you can get away with in so many MMOs these days. Especially in PvP and 5 man dungeons. Talking about positioning AoE areas so the Thief can fall back, switch to pistols and throw in some Bullets that Heal.

This upped communication requirement also makes me think about something that I've not seen ANet mention: In-game voice chat. But that's a different thread.